1. Field of the Invention
The invention relates to a starter motor assembly for starting an engine and, more particularly, to a starter motor assembly for starting a vehicle engine, the starter motor assembly having a non-load-bearing overrunning clutch.
2. Description of the Related Art
Starter motor assemblies to assist in starting engines, such as engines in vehicles, are well known. The conventional starter motor assembly broadly includes an electrical motor and a drive mechanism. The electric motor is energized by a battery upon closing of an ignition switch. The drive mechanism transmits the torque of the electric motor to the flywheel of the engine, thereby cranking the engine until the engine starts.
In greater detail, closing of the ignition switch (typically by turning a key) energizes a solenoid. Energization of the solenoid moves a metal solenoid shaft, resulting in the closing of electrical contacts, applying current from the battery to an armature of the electric motor. The motor's armature shaft subsequently rotates at a high speed. A planetary gear assembly reduces the speed of rotation of the armature shaft, and an output shaft rotates at a reduced speed.
The output shaft typically is coupled to an inner ring of an overrunning clutch. (Alternately, if no planetary gears are provided, the armature shaft is coupled directly to the drive ring of the clutch.) In one conventional clutch, called a load-bearing overruning clutch, the inner ring fits within an outer ring, and the outer ring is coupled to one end of a drive shaft. The other end of the drive shaft is attached to a pinion, which is coupled to the engine flywheel.
Rotation of the output shaft causes the inner ring to lock in place within the outer ring. Torque is then transmitted to the outer ring, and via the outer ring to the drive shaft and the pinion, and the engine is cranked. Subsequently, when the engine begins to run, the flywheel rotates the pinion and drive shaft faster than the armature rotates. This high speed rotation unlocks the clutch shell and the drive ring. These clutch components are thereby free to rotate relative to one another. The high speed rotation is not transmitted by the drive ring back to the armature shaft.
Alternatively, in a load-bearing clutch, the output shaft may be coupled to the outer ring, and the inner ring may be coupled to the drive shaft.
Non-load bearing overruning clutches have also been explored. In these types of clutches, the planetary gear assembly may include a ring gear surrounding the planetary gears. An outer clutch ring is made integral with the ring gear, projecting axially away from the planetary gears. An inner clutch ring is provided within the inner circumference of the outer clutch ring. In one version, the inner clutch ring has been fixed to an internal bracket that also rotatably supports the drive shaft, and the outer clutch ring locks up against the inner ring, or rotates freely to absorb the high speed engine rotation. In another version, the outer ring is fixed against the housing, and the inner clutch ring is made integral with the ring gears of the planetary gear assembly.
Such starter motors assemblies can be either "biaxial" or "coaxial." These terms relate to the location of the solenoid and solenoid shaft with respect to the armature shaft. In a biaxial starter motor, the solenoid and solenoid shaft are attached to the motor casing, with the solenoid shaft spaced away from and generally parallel to the armature shaft. In a coaxial starter motor, the solenoid is typically placed in the motor casing so that the solenoid shaft is aligned with the armature shaft. The coaxial assembly is considered to be more compact and universally adaptable than the biaxial assembly.
The conventional starter motor assemblies described above suffer from several disadvantages.
In the conventional starter motor assemblies having load-bearing clutches, use of the clutch to transmit force can wear out the clutch mechanism, causing the clutch to fail completely or causing portions of the clutch to break off and cause further damage to the starter motor assembly.
There are also problems in the conventional starter motor assemblies having non-load-bearing clutches. Electric motors can rotate either clockwise or counterclockwise. However, the clutch mechanisms described above, with one clutch ring fixed to a planetary gear assembly ring gear, and another clutch ring fixed to a center bracket can only work with a motion rotating in one direction. Furthermore, the conventional starter motor assemblies include pinion housings which have integral flanges that cover the clutch components, preventing or hindering their removal. Hence, these clutch mechanisms must be manufactured separately for clockwise rotation motors and counterclockwise rotation motors. This requirement increases the cost to manufacture the clutches.
Finally, alignment of the armature shaft and the drive shaft is difficult. It is typical in conventional starter motor assemblies, due to inherent manufacturing variations, that the armature shaft and drive shaft are slightly misaligned, which can lead to excessive uneven wear of the planetary gears. Typically, in order to help with the alignment, a bracket is provided to rotatably support the drive shaft, which increases the manufacturing cost, or a "nose" is provided on the housing, which makes the starter motor assembly bulky.